Resizing image resolution in ArcGIS Desktop?

The operative word is Resample, the version link is provided, similar links exist for prior versions of Arcmap. Also be aware of resampling issues associated with the resampling method and the nature of the data being assessed.

EDIT If you want to go the reverse route (ie decrease resolution) see Aggregate or Resample but be careful to choose your options based upon your input data type.

I would like to add Block Statistics as another method to alter the resolution of a raster. Depending upon your specific goals, Block Statistics allows fine control of how pixels are assigned based on:

• A user defined neighborhood (e.g. rectangle, circle, wedge etc)
• The type of statistics calculated within each block (e.g. mean, majority, variety etc).

In this example, pixel values are assigned based on a 3 X 3 rectangular window using "maximum" statistics.

The correct direction to alter resolution is from 250m to 1000m (finer to coarser resolution) and not the other way around.

For this, the Resample, Aggregate and Block Statistics tools are the appropriate alternatives (see user681's answer and Aaron's answer). Each of these tools offer different methods to summarize data, which I recommend reading whuber's answer in Resample binary raster to give *proportion* within new cell window.

However, if one really wants to go from a coarser resolution to an artificial finer one, a quick way to do this is:

1. Right click on the raster dataset to be exported, select “Data”, and then, select “Export data”.

It will open the Export Raster Data dialog:

2. Define the new pixel resolution (Cell Size).
3. Set the location, name and format of the file that will be created.
4. Check if the raster properties are in agreement with what was expected (including the Coordinate Reference System) and save it.

The resampling method used with this tool is the nearest neighbour (did not find an official documentation on this, but came to this conclusion upon making some tests).

It will appear the resolution has not changed because all values from the new dataset within a pixel from the original dataset will be the same. Even upon zooming and measuring the pixel won't be possible to distinguish smaller pixels in the new dataset. On the other hand, if one checks the layer properties, and its source tab, the Cell Size and number of rows and columns are altered. Another way to verify the resolution has changed is by converting the raster dataset to an XYZ table.